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How can a solid wall contain routes for water?
A cement-based wall contains solid phases and spaces that may hold water or air.
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For the named chemistry example, tricalcium silicate, Ca₃SiO₅, reacts with water and forms calcium silicate hydrate and calcium hydroxide. Calcium and silicon belong to compounds; there are no droplets of elemental calcium or silicon inside the binder.
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Explore the model (optional)
EXPLORE THE MODEL
A bounded diffusion profile
Inspect the structure; select a site or component for its label.
What this representation means
Assumed transport coefficient and fixed surface concentration; initially zero interior concentration. No reactions, binding, convection, finite back face or aging. This does not predict a concrete structure’s service life.
- Left → right
- Surface → 30 mm depth
- Layer color
- Interior slab midpoint samples of the graph
- Geometry
- Concentration slabs, not a measured pore image
C / Csurface = erfc[x / (2√Dt)]
Read sampled numerical values
| Depth below exposed face (mm) | Normalized concentration |
|---|---|
| 0 | 1 |
| 3 | 0.388 |
| 6 | 0.085 |
| 9 | 0.01 |
| 12 | 5.600e-4 |
| 15 | 1.612e-5 |
| 18 | 2.277e-7 |
| 21 | 1.565e-9 |
| 24 | 5.208e-12 |
| 27 | 8.356e-15 |
| 30 | 6.446e-18 |
The concentration is prescribed at the exposed surface and falls with depth. Increasing elapsed time or D spreads this ideal profile farther into the material. A diffusion length is not a crack depth or a failure boundary.
What this model can establish
Assumed transport coefficient and fixed surface concentration; initially zero interior concentration. No reactions, binding, convection, finite back face or aging. This does not predict a concrete structure’s service life.
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Optional: check your understanding
MAKE THE CONNECTION
What does the calcium in this binder represent?